The present invention relates in general to a power and communication (control) transmission system for use with an enclosed fluid apparatus such as mud filled drill string of an oil well or other fluid filled pipe arrangement. Although the primary use of the system of this invention is for powering downhill oil well drilling tools or instruments associated with the drill string, the system may also be used in association with any other type of enclosed fluid system. Most of the detailed description, will, however, be directed to the use of the system in the drilling for oil.
Reference is now made to my prior U.S. Pat. No. 4,215,426 which shows a telemetry and power transmission system for enclosed fluid systems, particularly for the petroleum drilling industry.
Because the mechanism for power propagation of the present invention is by means of sonic waves, the following analysis reveals the attenuations of the sonic waves in the drill string for various acoustic frequencies and drill string lengths. The following analysis will lead to certain improvements that can be brought about to improve the construction of the apparatus described in my prior U.S. Pat. No. 4,215,426.
Accordingly, it is an object of the present invention to provide an improved power transmission system for an enclosed fluid apparatus showing in particular improved constructions over my prior U.S. Pat. No. 4,215,426.
Chart 1 illustrates the approximate power an acoustic pressure wave can propagate in freshwater. The power of a sonic plane wave per unit of area of the surface of the plane wave is equal to the impedance of the fluid multiplied by the square of the pressure amplitude. The power propagated is independent of frequency.
CHART 1 ______________________________________ Pressure (psi) Watts/sq. in. (RMS) ______________________________________ 100 343 75 193 50 86 25 21 10 3.4 ______________________________________
If the attenuation of the sonic wave due to viscous damping is minimal, then the power propagation of the sonic wave is substantial for the purpose of the apparatus shown in U.S. Pat. No. 4,215,426.
The attenuation of the sonic pressure wave in the drill string results almost entirely for the viscous damping of the mud. Refer to B. J. Patton, "Development And Successful Testing of a Continuous-Wave, Logging-While-Drilling Telemetry System," JPT (October, 1977) pg. 1215-1221. Viscous losses are basically proportional to the inverse of the acoustic frequency squared. Refer to D. G. Tucker, and Gazey, B. K.: "Applied Underwater Acoustics," Pergamon Press Ltd., London, 1966. The theoretical half-depth (D) attentuation of mud with various mud kinematic viscosities and various acoustic frequencies is shown in FIG. 4. The complimentary equation for pressure amplitude loss over a drill string length is: ##EQU1## L is the drill string length D is half depth loss per 1000 ft.
(Ps)i is the pressure pulse source amplitude PA1 Ps is the pressure pulse amplitude a depth
For various acoustic frequencies Chart 2 gives the power attenuations in decibels and pressure amplitude attenuation in magnitude. The chart uses worst case for mud kinematic viscosity and worst case for depth, 30,000 feet.
CHART 2 ______________________________________ Frequency (hz) (Ps)i/(Ps) 20 log ((Ps)i/(Ps)) ______________________________________ 24 8,388,608 138.4 db 12 102 40 db 10 24.2 27.6 db 8 7.26 17.14 db 6 3.24 10.2 db ______________________________________
It is now apparent that a practical acoustic frequency for energy propagation without substantial loss, at a worst case situation of 30,000 fee of depth and high mud viscosity, is less than 12 hertz. This desire for low frequency acoustics opposes some physical length problems with respect to the acoustic energy converter in particular in my prior U.S. Pat. No. 4,215,426.
Chart 3 gives the lengths of the apparatus described in U.S. Pat. No. 4,215,426 for various low acoustic frequencies and a propagation velocity of 4300 ft/sec. The lengths on the chart are in feet, the normalized number of drill pipe sections (30 ft), and the normalized number of drill string sections (90 ft).
CHART 3 ______________________________________ freq. (hz) Length (ft) Drill pipe (30 ft.) Drill string (90 ft.) ______________________________________ 24 89.58 3 1 12 179.16 6 2 10 215 8 3 8 268.75 9 3 6 358.33 12 4 ______________________________________
The previous discussion concerning acoustic attenuation of sonic pressure waves as a function of frequency suggests that the apparatus disclosed in my U.S. Pat. No. 4,215,426 should operate below 12 hertz to minimize overwhelming acoustic attenuation under a worst case situation of 30,000 ft. and maximum mud viscosity. At these low acoustic frequencies the apparatus of my patent would have tremendous capabilities; namely, large power propagation to downhole tools via acoustics and high tool information propagation via electromagnetics as discussed in U.S. Pat. No. 4,215,426. Unfortunately, because the oscillating bodies described in the patent of the acoustic energy converter are distributed over its length and the length of the acoustic energy converter increases with decreasing frequency, the acoustic energy converter shown in the patent would be difficult to engineer and manufacture at these low frequencies. In this respect refer to Chart 3.
Accordingly, it is an object of the present invention to provide an improved power transmission system having improved engineering and manufacturing practicality in the construction of the acoustic energy converter and the acoustic energy generator.
Another object of the present invention is to provide an improved power transmission system for enclosed fluid apparatus having an improved acoustic energy converter or generator particularly adapted for very low acoustic frequencies, below 12 hertz.
To accomplish the foregoing and other objects of this invention there is provided either an acoustic energy converter or acoustic energy generator that is comprised of two channels (sonic wave tuning channels), one end of each channel being open to the drill string annulus and the other end of each channel being exposed to the opposite faces, respectively of the acoustic energy converter mechanism. Because the ends of the channels are open to the drill string annulus, the channels are filled with the same mud which is in the drill string annulus and therefore the acoustic characteristics of the channels to the sonic wave, particularly propagation velocity and wavelength, are similar to the drill string annulus. Energy is absorbed from the sonic wave traveling down the annulus of the drill string by these openings and sonic waves of the same frequency are created in each channel. The sonic waves in each channel propagate in opposite directions; essentially propagating toward the faces of the acoustic energy converter mechanism. By manipulating the channel lengths, one can design the overall length of the acoustic energy converter to be less than, equal to, or greater than one-half wavelength but still expose the faces of the acoustic energy converter mechanism to sonic pressure waves which are 180.degree. out of phase. The channels may be formed by spiralling or rippling or both spiraling and rippling thereof in the walls of the drill pipe. Complete embodiments of the invention are described hereinafter.